The explanation is that that kind of post doesn't provide as much "engagement" (i.e. likes, shares and in the end ad revenue) as the kind of posts that makes facebook a worse place (click-bait kind of articles, "fake news", gif memes etc).

Something like this happened to me too (though it was about a newborn baby). FB simply decided that some of my contacts are not worth showing up in my feed. Some other instead keep popping up, despite me clicking "Hide post - See fewer posts like this" like mad.